This invention relates to driving mechanisms.
In motor vehicles it is often required that engine cooling fans be driven at a speed which increases fairly uniformly with engine speed up to a predetermined engine speed, and which thereafter is maintained substantially constant as the engine speed increases above the said predetermined speed.
Heretofore it has been proposed that this be achieved by providing a driving mechanism having a drive part and a driven part with a shear-type fluid coupling therebetween, the shearing action of the fluid coupling being arranged to permit the required degree of slip between the parts when the engine speed is above the said predetermined speed.